RF transmission line structures oftentimes include opposing boundary walls between which electromagnetic or RF energy is intended to propagate. Types of RF transmission line structures include open parallel-plate, waveguide and resonant cavity based structures, for example. Frequently the RF transmission line structures are combined with a feed structure configured to introduce RF energy into an area between the opposing boundary walls in order to efficiently and effectively illuminate the RF transmission line structure, tailored to the desired phase and amplitude distribution. Most often, efficient launching or illumination of the RF energy with well-behaved coherency (uniform phase illumination) over a broad operating frequency bandwidth is desired.
Current practice for feeding parallel-plate and waveguide-based planar array type RF transmission line structures include: inscribed square/rectangle feed architecture wherein a line-feed or a linear array of couplers (waveguide- or coax-based feed-points oriented along a single line) launch a coherent internal plane-wave that illuminates a generally rectangular region (but leaves exterior regions outside the inscribed rectangular region, but inside the circular boundary, generally un-illuminated/wasted;) discrete perimeter feed architectures which use individual elements or groups of elements oriented along the array perimeter in order to feed a larger proportion of the circular region, but generally support only narrow operating frequency bands and require complex and difficult to package waveguide feeds and launches/transitions in order to provide the requisite phase coherency; and direct-fed waveguide slot antennas wherein a separate complex (rear-mounted) corporate and/or standing-wave-fed waveguide feed is employed to coherently illuminate the desired circular antenna shape in a “scalloped” pseudo-circular form-factor.
Notably, in open parallel-plate planar array antenna applications, for example, it is often desired to shape the antenna in a circular or near-circular (elliptical) shape. Examples include planar array surrogates for circular or elliptical parabolic dish antennas (for satellite communication, terrestrial point-to-point communication, radar systems, etc.) However, traditional waveguide-based feed architectures, by their nature, are generally rectilinear in nature and are therefore challenged to efficiently feed a circular shape. An inscribed-square geometrically fills only 64% of a circular area and due to finite limitations, it is generally not possible to feed the antenna all the way to its physical perimeter (i.e. “practical” inscribed-square efficiencies are typically less than 60%.)
Generically, the planar array antennas in circular or elliptical form-factors are generally fed via a separate rear-mount (direct-fed waveguide slot antennas) wherein a separate complex (rear-mounted) corporate and/or standing-wave-fed waveguide feed is employed to coherently illuminate the desired circular antenna shape in a “scalloped” pseudo-circular form-factor. Such arrays are inherently limited to narrow frequency-band operation and the bulk and packaging complexity associated with the (typically-multi-level) waveguide corporate feed adds undesired weight and cost.
In the special case of parallel-plate transmission-line based planar array antennas such as the Continuous Transverse Stub (CTS) array and Variable Inclination Continuous Transverse Stub (VICTS) array, current state of the (feed) technology has been traditionally to utilize (in ascending order of increased area efficiency and increased cost/complexity) a single linear-feed (“inscribed square/rectangle”;) or multiple parallel linear-feeds (“stepped feed”;) or multiple subarrays (“modularized feed”;) or via discretely-fed perimeter feed slots (“perimeter slot feed”.) While these approaches have varying levels of area-efficiency effectiveness, all suffer from the common inability to completely fill the entire circular extent of the antenna array and (particularly in the case of the latter more complex structures) significantly increase complexity and cost while limiting overall operating frequency bandwidth.
FIG. 1 illustrates a typical “inscribed square” feed methodology wherein a single waveguide line-feed 10 represents a linear RF source which coherently launches propagating parallel-plate electromagnetic waves 12 within a bounded parallel-plate region 14 and generally emanating at an angle normal/orthogonal to an axis 16 of the feed 10. The parallel-plate region 14 has a circular form factor, and the line-feed 10 illuminates a square-shaped or rectangular-shaped region 20 inscribed within the available circular region. Geometrically, this approach excites 64% of the available area, but in practice this figure is generally lower due to practical limitations on the physical extent of the line-feed 10.
FIG. 2 illustrates a variant of the inscribed square of FIG. 1, wherein multiple rectangular regions of propagating parallel-plate waves 12 are created, each fed by its own dedicated single waveguide line-feed 10. This method can provide marginally higher area efficiencies as compared to the inscribed-square, but at the expense of significantly higher component count and overall packaging complexity. In addition the foreshortened length of the wave/mode paths within each rectangular region can result in unintended consequences, for example constraints on antenna radiator coupling as well as undesired antenna sidelobe artifacts associated with the imperfect “blending” (discontinuities) between adjacent regions in the case of a planar array antenna.
A further extension of the rectangular approach (not shown) is known, wherein the feed is “modularized” into individual subarray regions with their own corresponding feeds. Such extension has the benefit of added area efficiency (filling of the available circular form factor) but again at the expense, for example, of antenna radiator coupling and sidelobe degradation in the case of a planar array antenna.
FIG. 3 illustrates a “Perimeter Discrete” feed method wherein individual feed elements 22 are introduced along the perimeter (in this case the left half) of the circular form factor of the parallel-plate region 14. The individual feed elements 22 launch the propagating parallel-plate waves 12 across the left half, and (as an option) a waveguide line-feed 10 located in the middle of the circular form factor launches the parallel-plate waves 12 across the right half. Again, this method realizes good improvement in area efficiency (fill-factor), but with substantial added feed network complexity for the individual feed elements 22. In the case of a planar array antenna type RF transmission line structure, again there is associated antenna sidelobe degradation.
In view of the above-noted shortcomings, there is a strong need in the art for an RF device which includes a more efficient feed arrangement for illuminating an RF transmission line structure in the case of a non-rectilinear form factor.